Method for digesting spent ion exchange resins and recovering actinides therefrom using microwave radiation

Abstract

The present invention relates to methods for digesting diphosphonic acid substituted cation exchange resins that have become loaded with actinides, rare earth metals, or heavy metals, in a way that allows for downstream chromatographic analysis of the adsorbed species without damage to or inadequate elution from the downstream chromatographic resins. The methods of the present invention involve contacting the loaded diphosphonic acid resin with concentrated oxidizing acid in a closed vessel, and irradiating this mixture with microwave radiation. This efficiently increases the temperature of the mixture to a level suitable for digestion of the resin without the use of dehydrating acids that can damage downstream analytical resins. In order to ensure more complete digestion, the irradiated mixture can be mixed with hydrogen peroxide or other oxidant, and reirradiated with microwave radiation.

What is claimed is: 
     
       1. A method for releasing actinide, rare earth, or heavy metal elements from a diphosphonic acid-substituted ion exchange resin, comprising: (A) contacting a diphosphonic acid-substituted ion exchange resin having actinide, rare earth, or heavy metal elements adsorbed thereon with a low boiling point oxidizing acid in a closed microwave vessel to form a mixture; and   (B) irradiating the mixture with microwave radiation for a sufficient time and at a sufficient energy, so that the mixture reaches a sufficient temperature to dissolve at least a portion of the resin to form a first digested mixture which releases the actinide, rare earth or heavy metal elements from the diphosphonic acid-substituted ion exchange resin.   
     
     
       2. The method according to claim 1, further comprising: (C) cooling the first digested mixture;   (D) adding hydrogen peroxide to the first digested mixture to form a second mixture; and   (E) irradiating the second mixture with microwave radiation for a sufficient time and at a sufficient temperature to dissolve at least a portion of the second mixture.   
     
     
       3. The method according to claim 1, wherein the low boiling point oxidizing acid is nitric acid. 
     
     
       4. The method according to claim 3, wherein the nitric acid has a normality of about 12 to about 16. 
     
     
       5. The method according to claim 1, wherein the temperature of the irradiated mixture is from about 170° C. to about 250° C. 
     
     
       6. The method according to claim 5, wherein the irradiation is carried out for a time between about 10 minutes and about 40 minutes. 
     
     
       7. The method according to claim 1, wherein the diphosphonic acid-substituted ion exchange resin comprises a polystyrene/divinylbenzene backbone substituted with diphosphonic acid groups. 
     
     
       8. The method according to claim 7, wherein the diphosphonic acid-substituted ion exchange resin is further substituted with sulfonic acid groups. 
     
     
       9. The method according to claim 2, wherein the hydrogen peroxide added has a concentration of about 30 wt %, based on the hydrogen peroxide composition. 
     
     
       10. A method for releasing actinide, rare earth, or heavy metal elements from a diphosphonic acid-substituted polystyrene/divinylbenzene ion exchange resin, comprising: (A) contacting a diphosphonic acid-substituted polystyrene/divinylbenzene ion exchange resin having actinide, rare earth, or heavy metal elements adsorbed thereon with nitric acid having a normality of about 12 to about 16 in a closed microwave vessel to form a mixture;   (B) irradiating the mixture with microwave radiation for about 10 minutes to about 40 minutes at an energy sufficient to obtain a mixture temperature of about 170° C. to about 250° C. and dissolving at least a portion of the resin to form a first digested mixture comprising organic digestion products;   (C) cooling the first digested mixture;   (D) adding hydrogen peroxide having a concentration of about 30 wt %, based on the hydrogen peroxide composition, to the first digested mixture to form a second mixture; and   (E) irradiating the second mixture with microwave radiation for a sufficient time and at a sufficient temperature to digest said organic digestion products which releases the actinide, rare earth or heavy metal elements from the diphosphonic acid-substituted ion exchange resin.   
     
     
       11. A method for determining the identity and concentration of actinides, rare earth, or heavy metal cations present in a sample, comprising: (1) dissolving or diluting the sample in water to form a sample solution;   (2) contacting the sample solution with a diphosphonic acid-substituted ion exchange resin under conditions sufficient to adsorb actinide, rare earth, or heavy metal cations thereon;   (3) removing the diphosphonic acid-substituted ion exchange resin from contact with the sample solution;   (4) releasing the actinide, rare earth, or heavy metal cations from the diphosphonic acid-substituted ion exchange resin; by (A) contacting the diphosphonic acid-substituted ion exchange resin having actinide, rare earth, or heavy metal cations adsorbed thereon with a low boiling point oxidizing acid in a closed microwave vessel to form a mixture; and   (B) irradiating the mixture with microwave radiation for a sufficient time and at a sufficient energy so that the mixture reaches a sufficient temperature to dissolve at least a portion of the resin to form a first digested mixture;     (5) analyzing the actinide, rare earth, or heavy metal content of the first digested mixture by column chromatography.   
     
     
       12. The method according to claim 11, wherein the sample is selected from the group consisting of a soil sample, a urine sample, a tissue sample, a fecal matter sample, a cooling water sample, and a wastewater sample. 
     
     
       13. The method according to claim 11, wherein the diphosphonic acid-substituted ion exchange resin comprises a diphosphonic acid-substituted polystyrene/divinylbenzene polymer that is optionally further substituted with sulfonic acid groups. 
     
     
       14. The method according to claim 13, wherein the releasing of the actinide, rare earth, or heavy metal cations in (4) comprises: (A) contacting a diphosphonic acid-substituted polystyrene/divinylbenzene ion exchange resin having actinide, rare earth, or heavy metal cations adsorbed thereon with nitric acid having a normality of about 12 to about 16 in a closed microwave vessel to form a mixture;   (B) irradiating the mixture with microwave radiation for about 10 minutes to about 40 minutes at an energy sufficient to obtain a mixture temperature of about 170° C. to about 250° C. and dissolving at least a portion of the resin to form a first digested mixture comprising organic digestion products;   (C) cooling the first digested mixture;   (D) adding hydrogen peroxide having a concentration of about 30 wt %, based on the hydrogen peroxide composition, to the first digested mixture to form a second mixture; and   (E) irradiating the second mixture with microwave radiation for a sufficient time and at a sufficient temperature to digest any remaining organic digestion products.